A Two-stage Thermal Evolution Model of Magmas in Continental Crust

Abstract

When a basaltic magma is emplaced in a continental crust, a silicic magma is generated by melting of the crust. The light silicic magma forms a separate magma layer with little chemical interaction with the underlying dense basaltic magma layer. Extensive melting occurs at the boundary between the silicic magma and the crust while the basalt acts a heat source. The mass and heat transfer at the boundary between the silicic magma and the crust controls the thermal evolution of the silicic magma. The thermal evolution of the silicic magma after the basalt emplacement is divided into two stages. In the first stage, the temperature in the silicic magma rises above and then decays back to the melting temperature of the crust on a short timescale (102 years). The results of fluid dynamics experiments suggest that the silicic magma generally has a lower melting temperature than the crust because of fractional crystallization and mixing of partial melts during the first stage, and that it can be effectively liquid at the end of the first stage. In the second stage, the silicic magma cools slowly by heat conduction on a much longer timescale (105 years). Petrological features of the magma in the second stage are strongly constrained by petrological features of the surrounding crust as well as those of the supplied magma itself; its temperature remains at or just below the melting temperature of the crust for a long time because of the slow cooling rate; its phenocryst content reflects the difference in the melt fraction vs temperature relationships between the magma and the crust. Judging from the distinct cooling rate between the two stages, erupted magmas are statistically more likely to reflect the characteristics of magmas in the second stage.